Tether USD (USDT) is a stablecoin pegged to the US Dollar. The value is meant to mirror the value of the U.S. dollar. Tether tokens are seen as a stable store of value in an otherwise highly volatile crypto-currency ecosystem.
Tether is controlled by Tether Limited. USDT tokens are minted or burnt (destroyed) as the reserves held by Tether Limited increases (via deposits) or falls (via claims against the tokens).
Tether is the first stablecoin to be created and is the most popular stablecoin used in the ecosystem. It is the largest stablecoin by circulation and market capitalization.